This application claims priority from EP 01302573.9 filed Mar. 20, 2001, herein incorporated by reference.
The invention relates generally to lithographic apparatus and more particularly to methods of aligning planar motors used in a lithographic projection apparatus.
In general, a lithographic apparatus comprises a radiation system to supply a projection beam of radiation, a support structure to support patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table to hold a substrate, and a projection system to project the patterned beam onto a target portion of the substrate.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to structure or means that can be used to endow an incoming radiation beam with a patterned cross section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being create d in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on such mirror arrays can be gleaned, for example, from United States Patents U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in United States Patent U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at one time; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In lithographic apparatus, planar motors have been proposed for use in positioning systems for the object tables, especially the substrate table. A suitable planar motor for positioning the substrate table(s) comprises a so-called xe2x80x9cchecker-boardxe2x80x9d of magnets over which a coil unit (sometimes also referred to as an armature or translator) moves in response to the forces exerted on it as the coils are energized. The checker-board, forming the stator of the motor, comprises rows and columns of magnets forming a square array; each magnet generates a vertical magnetic field but the directions alternate. Referring to the checker board analogy, the black squares, say, are magnets with a north pole uppermost and the white squares are magnets with a south pole uppermost. The stator may also include further magnets, generating fields in the plane of the board, arranged in between the rows and columns of the primary magnets. These further magnets can be arranged to enhance the field generated by the primary magnets and also to provide levitation forces to form a bearing for the coil unit.
Unlike a simple rotary motor, the magnitude and direction of the force exerted by a planar motor in response to a given energizing current depends on the physical position of the translator coil(s) relative to the periodic magnet structure in the stator, which may be referred to as the translator phase position. Thus to determine the energizing current to be applied to the coil(s) to generate a desired force, it is necessary to know the translator phase position. In a lithographic apparatus, the position of a table, e.g. the substrate table, is conventionally measured using interferometric displacement measuring means which are exceptionally accurate, have large operating ranges and have very fast response times. However, most positioning systems have fine and coarse positioning units and the interferometers are used to measure the position of the fine stage relative to the projection lens, which is mounted on a reference frame isolated from the rest of the apparatus and particularly the positioning systems. Thus, the stage position as measured by the interferometer is not very useful in measuring the position of a translator of a planar motor used in the coarse positioning unit relative to a stator mounted to the main frame of the apparatus.
To determine the relative position of the translator and stator it is therefore necessary to provide an additional position sensor. Movements of the translator can be conveniently measured by an optical encoder but such a device measures only displacements and requires to be initialized. To determine the initial position it is possible to provide an additional position measuring system capable of directly measuring the absolute position of the table. (Such a system may not be suitable for measuring the position of the table throughout its range of movement due to a limited measurement range or slow response speed.) Alternatively, a physical stop can be provided at an extreme of the range of movement of the tablexe2x80x94the table is then driven against the stop from its initial unknown position. When movement of the table ceases it is known to be hard up against the stop, defining its position. The provision of an additional absolute position measuring system incurs additional expense and occupies space which can be at a premium in a lithographic apparatus. Repeatedly driving the table against a physical stop causes contamination, undesirable wear and shock to the table.
EP-0 297 642-A1 describes a method of alignment of a linear or rotary motor of the synchronous type in which the relations between the driving forces of the motor and the energizing currents in the phase windings are periodic functions of the rotor or translator position and that has an incremental encoder for measuring displacements of the rotor or translator. The method involves generating measuring currents in different phase windings in turn to produce vibration in the rotor or stator and determines the position of the rotor or translator from the amplitude of the induced vibrations.
In an aspect of at least one embodiment of the present invention, there is provided a method of determining the initial position of the moving part of a planar motor in a lithographic projection apparatus.
At least one embodiment of the present invention includes a lithographic apparatus comprising a radiation system for supplying a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate, first control means for energizing a plurality of said energizable coils in turn with an oscillating signal sufficient to cause vibrations of said translator having an amplitude less than the period of said periodic magnet structure, vibration measuring means for measuring said vibrations of said translator, and second control means for determining the phase relationship between said translator and said stator on the basis of said measured vibrations.
According to at least one embodiment of the present invention, there is also provided a positioning system for positioning an object, said positioning system comprising: a planar motor having a stator and a translator, one of said stator and said translator comprising a periodic magnet structure and the other of said stator and said translator comprising a plurality of energizable coils; a first control means for energizing a plurality of said energizable coils in turn with an oscillating signal sufficient to cause vibrations of said translator having an amplitude less than the period of said periodic magnet structure; a vibration measuring means for measuring said vibrations of said translator; and a second control means for determining the phase relationship between said translator and said stator on the basis of said measured vibrations.
According to at least one embodiment of the present invention, there is further provided a device manufacturing method using a lithographic projection apparatus comprising a radiation system for providing a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, a positioning system for positioning at least one of said support structure and said substrate table, said positioning system comprising a planar motor having a stator and a translator, one of said stator and said translator comprising a periodic magnet structure and the other of said stator and said translator comprising a plurality of energizable coils, a projection system for projecting the patterned beam onto a target portion of said substrate; the method comprising: providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system; using patterning structure to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material; energizing a plurality of said energizable coils in turn with an oscillating signal sufficient to cause vibrations of said translator having an amplitude less than the period of said periodic magnet structure; measuring said vibrations of said translator; and determining the phase relationship between said translator and said stator on the basis of said measured vibrations.
According to at least one embodiment of the present invention, there is also provided a lithographic projection apparatus comprising: a radiation system for providing a projection beam of radiation; a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern; a substrate table for holding a substrate; a positioning system for positioning at least one of said support structure and said substrate table, said positioning system comprising a planar motor having a stator and a translator, one of said stator and said translator comprising a periodic magnet structure and the other of said stator and said translator comprising a plurality of energizable coils; a projection system for projecting the patterned beam onto a target portion of said substrate; an array of optically detectable marks on the magnet structure; an optical detecting means for detecting the array of distinct optical marks; and a control means for determining the relative position of the stator and translator on the basis of the detected distinct optical marks.
Although specific reference may be made in this text to the use of the apparatus according to at least one embodiment of the present invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.
In the description below, reference is made to an orthogonal X, Y, Z coordinate system of which the Z direction may be referred to as vertical. However, this should not be taken as requiring a specific orientation of the apparatus. The notation Ri is used to denote rotation about an axis parallel to the I direction.